Thanks, i think i understand the concept of this but i have one more question.

What if the question was Cos theta = -1/ sqrt2 ?
The reference angle would be 60 degrees? and it would be at quadrant 2?
So 180-60= 120degrees BUT my textbook says 135degrees. What might be the proplem?

Oh yea for the previous question i asked you, why does sin thetha = - sqrt of 3 /2 belong to the third and fourth quadrant?

Originally Posted by mr fantastic

You should know that the reference angle is . Use the reference angle to get the required angle that lies in the third quadrant (book's answer) and also fourth quadrant (omitted by book ....)